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[AD&D] Why THAC0?

This thread has been started to prevent further derailment of this thread: [4.0] Insults intelligence.. Yeah, a bit edition war like, but THAC0 is actually a really interesting subject so I want to continue the discussion here. Much of the debate seems to hinge on two different definitions of Armour Class:

Originally Posted by D20

Armor Class A number representing a creature's ability to avoid being hit in combat.

Originally Posted by AD&D 2e

Armor Class (abbr. AC)--a rating for the protective value of a type of armor, figured from 10 (no armor at all) to 0 or even -10 (the best magical armor). The higher the AC, the more vulnerable the character is to attack.

I have edited the subsequent exchanges to keep closer to the subject matter, omitting comments that I thought extraneous. Anyway, off we go...

Originally Posted by Yakk

If you didn't motify THAC0 it wasn't that useful.

If you applied your strength modifier to your THAC0 and your magic weapon modifier to your THAC0, you reduce the math required to determine a hit to a single addition (or subtraction) of a single number.

Of course, you could do:
d20 + opponents AC + your to-hit bonuses
and see if it matched your THAC0. But that slowed the hit calculation down. :)

Nah, you apply all modifiers separately to avoid confusion. There is no speed difference in rolling a die and adding a modifier, but it depends how you do the actual in play event. I have heard of people saying stuff like "I hit AC 3", or something along those lines, instead of calling out their roll, in which case the calculation would need to be done before hand.

However, it can also work like this

Target Number = 21 (always)

Player: Rolls Dice, adds fighting ability and any other modifiers
Game Master: Adds armour class of target to the total, if it equals or exceeds 21 it is a hit.

The player can always know what AC he hits this way as well, of course [e.g. "19 isn't a hit? God damn, this thing has better than AC 2!"].

Alternatively, THAC0 eliminates fighting ability and becomes the target number, but all the player needs to do then is:

There is no difference in the second case, except transparency of the maths for the player [e.g. "15 isn't a hit? Hmmn, what's my THAC0 again? Oh yeah, 13, so it has better than AC −2?"]. However, the game master needs to have the unmodified THAC0 of all combatants written down in addition to their attack roll modifiers.

Originally Posted by Thane of Fife

There are, in fact, some things which explicitly modify THAC0, such as the Weapons vs Armor Table.

Personally, I've always just used Roll higher than THAC0 - Armor Class, but there are many ways to do it. Really, though, I taught myself to play from a D&D introductory box when I was 5, and it used THAC0, so it certainly isn't anywhere near as incomprehensible as some people make it out to be.

The second edition one? Yeah, I think that optional rule is about the only instance. The more I think about it, the more convinced I am that the diversity of methods of calculation are in fact what leads people to believe it is "counter intuitive". Indeed, THAC0 could stand for the entire second edition game in that respect.

Originally Posted by Talic

Reasons THAC0 was nonintuitive.

It was additive, rather than opposing. In other words, you added your opponent's AC to your attack roll. This essentially AC into more or less, a bonus/penalty to hit... And your skill as the goal. Shouldn't it be the other way around?

After all, the Armor Class, your opponent's defenses, are the obstacle. They're what you're trying to overcome. In the Thac0 system, you were trying to overcome your skill.

Contrast to 3.X, where Armor Class isn't a modifier, it's the final goal. In other words, it bases your hit ability around a single Armor Class, which is actually fairly rare. You needed to derive your actual success based on how hard it was to hit that one.

Now add in that armor that improved your AC by four lowered it, even while you added your AC to that attack roll. Magic swords also lowered your THAC0, but, due to the convoluted way it was set up, lowering your Thac0 makes it easier to hit, and lowering your AC makes it harder.

In other words, bonuses to hit modified the final difficulty, not the attack.

It's like saying that the wall is actually physically weaker because a master sniper is shooting at it... In the Thac0 system, you were modifying the target number, not your attack.

Do not not modify THAC0. That is why virtually everything is expressed as a modifier to the attack roll and not THAC0. The modifier is always the same [e.g. positive or negative], and that is why THAC0 works the way it does. Trying to modify THAC0 in AD&D is like trying to modify your opponent's armour class by your attack bonus in D20/3e.

Let's take an example:

A 5th level fighter with 17 strength has +1 to hit. He is also wielding a cursed sword, which is imposing a −1 to hit. In addition, his target is enjoying the effects of a prayer spell, imposing a further −1 to hit.

The player attacks his opponent and rolls a 13, which he modifies by −1 to get 12. The game master checks the armour class of his opponent, which is 5 and adds it to the result to get 17, which is better than his THAC0 of 16. A hit has been scored.

In this instance, the player was told there was a prayer effect in play and asked to modify his attack roll accordingly. If the game master had wanted to keep that secret, then he could have applied the same modifier to the armour class of the target.

Originally Posted by Talic

As opposed to the following:
A 5th level fighter with a 17 strength has a +9 to hit. He has a bane effect on him, imposing a -1 to attack rolls. In addition, his target has declared him a dodge target, which increases the target's AC by one.

The fighter rolls, and gets a 13. He looks to the DM, and says "I hit AC 22." (13+9) The DM compares it to the AC of the target (20), adds 1 for the dodge (player doesn't have that knowledge), and declares the hit.

In this way, the DM, as storyteller/mediator, is letting players control player information, and handling privately only the information that players are not privy to. He doesn't need to know the Thac0 of his players. Only the roll, and the modifiers that players are not privy to. Simple.

And yet, the DM still has to look at a player sheet, and a monster sheet, to determine the hit. The DM has to process the number and compare to a player number. Whereas, in 3.x, the DM compares the player number only against the privy information. Each player can handle more of the equation, and the DM is left mediating the information that isn't.

The only difference are that the game master almost always adds a modifier by way of armour class in AD&D, and that in D20 the modifiers are typically much larger. The trade off is that in AD&D negative modifiers are always good for the defender, and positive modifiers always good for the attacker.

If the game master needs to look at the player sheet it means he does not have their THAC0 already available, but he should have their THAC0s and that of all the combatants in play available, just like he should have their armour classes available in D20. That is not extra math, it is just like having BAB noted separately from AB.

Originally Posted by Talic

Well, that in every edition after thac0, bonuses added, and penalties subtracted. Rather than having to remember whether this is a bonus that added to this score, or it was a bonus that subtracted from that one. Counter-intuitive.

Again you misunderstand. You are assuming these terms:

My plusses are good for me
My minuses are bad for me
His pluses are bad for me
His minuses are good for me

In AD&D it is:

Plusses against him are good for me
Minuses against him are good for me
Plusses against me are bad for me
Minuses against me are bad for me

Neither is more intuitive than the other.

Originally Posted by Talic

Or, he has 17 creatures to keep track of already... Why not let the players handle their own bonuses, rather than ensuring that they will always have an unknown modifier in the form of AC... Which would be more easily expressed by making it the target number. That way, in many fights, the PC could handle most of the work. As is, the DM, in AD&D ALWAYS has to add in a modifier before comparing. Always. In 3.x and beyond? If there are no surprises? He can just look at the two numbers. This reduction in variables streamlines combat, and makes things move faster. That's a good thing.

Not always, of course, not on AC 0. However, I do not really think that you can make the case that there are typically less modifiers in D20 than AD&D.

Originally Posted by Talic

I assume:

Plusses are good for the roll.
Minuses are bad for the roll.

Plusses increase things.
Minuses decrease things.

AD&D assumes:
Plusses are good for the roll... unless they're to AC, when they're bad for the roll.
Plusses increase things... With the exception of this and that and the other.

Pluses to the roll in AD&D are good for the person rolling the die, minuses to the roll are bad for the person rolling the die. Same in D20.

Originally Posted by Talic

Intuitive is when Plusses add, and minuses subtract. Intuitive is when 5 is better than 4.
Intuitive is when the AC system doesn't start with a number as completely arbitrary as whether the MacGuffin is a book or a badger.
Thac0 cheerfully says no to all of these.
No, just that the ones that are there are not modifiers. The AC of your foe is the whole goal. That's the resistance you're overcoming. Beat the AC, accomplish the goal.

In football, if you overcome your opponent's defense? It's a touchdown.
In hockey or soccer? Goal.
But in AD&D, there is no overcoming defenses. There's overcoming your Thac0. The sports? Intuitive. The Thac0? NOT.

This initial idea that Armour Class is a number to be overcome, rather than a modifier to a roll. That is completely non intuitive, since "armour class" does not denote that at all except by learned exposure. If it were called "defence rating" you might have a point.

Originally Posted by Talic

A rose by any other name. Naming differences aren't the issue here. The fact that Armor Class combines (in all discussed editions):
Bonuses from armor
Bonuses from agility
Bonuses from magic
Miscellaneous other Bonuses

Shows that regardless of what you call it, what it actually IS, is a measure of a character's ability to dodge/block/parry or otherwise avoid being hit.

In other words, whether it's called armor class or defense rating, it IS a rating of the character's defense.

On what basis do you conclude that armour class is a number to be overcome when in fact it only is in D20? That is purely subjective on your part. In Chainmail, for instance, Armour Class is a number cross referenced against Weapon Class to find the probability of a kill on the man to man melee table.

Armour classification is just that, the class of armour. You add it to the attack roll (or deduct it from THAC0 if you prefer). This is the sort of statement that leads me to infer that you are are unfamiliar with the design approach of AD&D, in that you are imposing the conceits of D20 onto the game system and coming to an erroneous conclusion.

The above list of bonuses makes no case at all. These are all modifiers to the attack roll and not bonuses to a number to be overcome.

Originally Posted by Talic

Let's say I have a suit of armor. It's magical +1.
There's a bonus to AC from armor.
There's a bonus to AC from magic.
Now let's say I have an 18 dexterity.
There's a bonus to AC from agility.

All three of these apply in both 2ed and 3ed (and 4ed).
So NO. Armor Class is not just your class of armor. It also factors in your ability to dodge, at the very least.

That is actually wrong. Magic and dexterity do not give a bonus to armour class, they impose negative penalties on the attack roll. You can roll them into armour class (which is commonly done), but that is not the actual armour class (which is a number in AD&D used to find bonuses to hit versus weapon type).

Originally Posted by Talic

So it's more.
In other words, your assertation is incorrect. Because Armor Class is NOT "just that". Because your dexterity is not armor, even though it modifies armor class. Armor Class is an abstract concept, representing how hard you are to hit.

Yeah, it is just that. The "modified" armour class is a short handed rolling up of bonuses and penalties to make the game go more quickly, but armour class is literally class of armour.

Originally Posted by Talic

Incorrect. AGAIN. AC is a number. A Classification of armor? Then why did the AD&D tarrasque (who, as far as I can tell, was not wearing chain greaves) sport a -10?

No. Direct from the owners of the game:

That's not the Classification of armor. No, Armor class was a derived term, true. But not from AD&D. No. It was from a tactical Civil War naval combat game, when it actually did represent how well armored ships were.

Wrong, I am afraid. Armour Class is right there in Chainmail, but reversed so that higher is better. Arneson is reported to have taken inspiration from the Naval game, but armour class was already part of the Chainmail Combat system. There is a reason that hit charts in OD&D only go down to AC 2 despite the presence of magic items.

Originally Posted by Talic

But it means something different. Don't like that? Sorry. But the people who make the game disagree with you.

You're wrong.

Nope, the charts were extended in AD&D (and in Greyhawk) for ease of play, but the actual "rolling up" process is not armour class in its primary sense, though "armour class" is used to denote the secondary meaning in almost all short handed instances. The difference is important when you are dealing with the repeating "20" rule or when using weapon type versus armour class.

Originally Posted by Talic

I can't believe it. I show you verbatim word for word, from the owners of the game, text that directly contradicts you... And you STILL refuse to see it!?

Yeah, I'm done here. Have fun with your... Well, with whatever it is you do.

Here is the original wording:

Monsters & Treasure, p. 31:

ARMOR: Armor proper subtracts its bonus [Note: here we are talking magical bonus] from the hit dice of the opponents of its wearer. If the shield's bonus is greater than that of the armor there is a one third chance that the blow will be caught by the shield, thus giving the additional subtraction.

Originally Posted by Talic

That notes armor. Not armor class.

Show me some sort of specific text that states that Armor Class only measures the effectiveness of the armor that is worn, and counts in no other factors, or retract your point.

There are two types of armour class. There is:

a) The Armour Type's Armour Class.

Armour protection is measured by Armour Class (AC), a number rating; the lower the Armour Class number, the better the protection.

Armor Class (abbr. AC)--a rating for the protective value of a type of armor, figured from 10 (no armor at all) to 0 or even -10 (the best magical armor). The higher the AC, the more vulnerable the character is to attack.

b) The Modified Amour Class of the defender.

Defensive Adjustment applies to a character's saving throws (see Glossary) against attacks that can be dodged--lightning bolts, boulders, etc. It also modifies the character's Armor Class (see Glossary), representing his ability to dodge normal missiles and parry weapon thrusts. For example, Rath is wearing chain mail, giving him an Armor Class of 5. If his Dexterity score is 16, his Armor Class is modified by -2 to 3, making him harder to hit. If his Dexterity score is 5, his Armor Class is modified by +2 to 7, making him
easier to hit. (In some situations, beneficial Dexterity modifiers to Armor Class do not apply. Usually this occurs when a character is attacked from behind or when his movement is restricted--attacked while prone, tied up, on a ledge, climbing a rope, etc.)

The former is important for such things as weapon type versus armour class and the "repeating 20" rule in AD&D. It is also important for magic nullifying effects, and attacks from the flanks that negate benefits from dexterity or shield.

Now here is the exchange in question:

Originally Posted by Matthew

Originally Posted by Talic

Let's say I have a suit of armour. It's magical +1.
There's a bonus to AC from armour.
There's a bonus to AC from magic.
Now let's say I have an 18 dexterity.
There's a bonus to AC from agility.

All three of these apply in both 2ed and 3ed (and 4ed).
So NO. Armour Class is not just your class of armour. It also factors in your ability to dodge, at the very least.

That is actually wrong. Magic and dexterity do not give a bonus to armour class, they impose negative penalties on the attack roll. You can roll them into armour class (which is commonly done), but that is not the actual armour class (which is a number in AD&D used to find bonuses to hit versus weapon type).

I can see that this has caused some confusion. My intention was to distinguish here between two forms of armour class. The base armour class is determined by armour type (used for weapon type versus armour class adjustments, if used) and the short hand that rolls all modifiers into a defensive value also termed "armour class". Armour Type is not a bonus to "armour class", it is the "base armour class" ranging from 10 to 0. Take your example for instance:

A first level fighter needs "20" to hit that Armour Class in second edition, but if he has a +1 sword he doesn't suddenly need "19". He still needs "20" because the modified armour class deducts from his "to hit" roll. That's why armour itself only has a base classification system between 0 and 10.

Last edited by Matthew; 2009-06-06 at 01:24 PM.

It is a joyful thing indeed to hold intimate converse with a man after one’s own heart, chatting without reserve about things of interest or the fleeting topics of the world; but such, alas, are few and far between.

Re: [AD&D] Why THAC0?

I always thought the reason THAC0 existed was to keep information away from the players. The players weren't suppose to know the AC of the monsters they're facing. If I recall correctly (and I probably aren't), the original intention was that players aren't even suppose to know their own THAC0. There was a reason why the THAC0 chart was on the DM screen. The DM was suppose to be the only one to figure out the target "to hit" number based on the PC's THAC0 and the monster's AC because he was the only one with both pieces of information. THAC0 was there to maintain the illusion so the PCs can't "peek under the hood" so to speak. The only thing the PCs were suppose to know after rolling their dice was "hit or miss".

This is the only reason I could think of to explain the roundabout nature of THAC0.

Re: [AD&D] Why THAC0?

If I'm not mistaken, in original 1e D&D, there was no thaco or anything like it. Instead, you rolled a d20 and consulted a table which had every single monster on one axis and the player character levels on the other. I'm not 100% sure on the specifics.

What I do know is that when 2e came around, THAC0 was developed as an abstraction of this graph. Because they operated on the same function as the chart, THAC0 and AC end up decreasing as you become more formidable.

Re: [AD&D] Why THAC0?

Originally Posted by Gaurd Juris

If I'm not mistaken, in original 1e D&D, there was no thaco or anything like it. Instead, you rolled a d20 and consulted a table which had every single monster on one access and the player character levels on the other. I'm not 100% sure on the specifics.

What I do know is that when 2e came around, THAC0 was developed as an abstraction of this graph. Because they operated on the same function as the chart, THAC0 and AC end up decreasing as you become more formidable.

Yeah, that's what I thought. When the math behind THAC0 and AC was created, it was information that should only have been available to the DM. The way THAC0 and AC works makes perfect sense if the DM was the only one with the information.

The player was suppose to roll the dice, add/substract any modifiers and give that number to the DM. The DM determines the target "to hit" from the PC's THAC0 and the monster's AC (or looks up the chart from 1e) and tells the player whether he hit or missed. The player was never suppose to do anything with his own THAC0. He was suppose to be able to play perfectly fine without even know what his own THAC0 was.

Re: [AD&D] Why THAC0?

I always thought the reason THAC0 existed was to keep information away from the players. The players weren't suppose to know the AC of the monsters they're facing.

That's still the case. In 3.x, a player can roll the die and add all appropriate modifiers, and say "I got a 19". The DM then looks at the monster's AC, sees whether it's higher than 19, and tells the player whether the attack hit. The player doesn't need to know the monster's AC any more than in 2nd edition. You might be able to figure it out ("Wait, last round, I missed on an 18, and this round I hit on a 19"), but you could do that in 2nd, too.

Ultimately, using THAC0 gives exactly the same effect as the system in 3.x, but the 3.x system is simpler and easier to use. OK, maybe it's only a little bit simpler, but it does have an advantage over THAC0, and I've never seen anyone argue that THAC0 has any advantage at all.

Time travels in divers paces with divers persons.
—As You Like It, III:ii:328

Re: [AD&D] Why THAC0?

Originally Posted by Chronos

That's still the case. In 3.x, a player can roll the die and add all appropriate modifiers, and say "I got a 19". The DM then looks at the monster's AC, sees whether it's higher than 19, and tells the player whether the attack hit. The player doesn't need to know the monster's AC any more than in 2nd edition. You might be able to figure it out ("Wait, last round, I missed on an 18, and this round I hit on a 19"), but you could do that in 2nd, too.

But the 3.x player knows his own BAB. He can figure out that the monster's AC was 19. If the 2e/1e player weren't suppose to know his own THAC0, then even if the player hit on a 19, he can't figure out what the real AC of the monster was because he doesn't know his own THAC0.

The modified roll is 15. You were targeting an AC 0 thing and your THAC0 was 15, so you hit.

THAC0 was all addition for the algebra.

All it was was THAC0 giving you the target numbers for anything you would ever come across. You just had to modify the dice roll to compare to that number along the number line. That number line represents all the AC range.

THAC0-AC=target number

15-0=15

If the AC was 2

15-2=13

If the AC was -2

15-(-2)=X
15+2=17

Thac0 15 needs 17 rolled to hit AC -2.

That is what THAC0 tells you, the target numbers for ACs. A player need only know his known modifiers to tell what the roll could have hit. Then any hidden modifiers the DM would add after the AC capable of being hit could be figured in.

Ages 12 and up usually means 6th grade, and by then isn't every taking basic algebra still? So where has the problem been with THAC0?

Originally Posted by Gaurd Juris

If I'm not mistaken, in original 1e D&D, there was no thaco or anything like it. Instead, you rolled a d20 and consulted a table which had every single monster on one axis and the player character levels on the other. I'm not 100% sure on the specifics.

Re: [AD&D] Why THAC0?

At a certain point, it became assumed that players knew their own THAC0. THAC0 had the biggest box on many a TSR character sheet, IIRC. This was true even in AD&D.

There was nothing wrong with THAC0. D20 makes a minor improvement, making hitting stuff slightly more intuitive. It's not massively better, but it is a little bit better. Hm, I guess there is one substantive improvement. In AD&D/2e -10 was an arbitrary floor for AC, and that never made much sense to me. D20 did away with that (though many tables had already house ruled it away anyway).

Re: [AD&D] Why THAC0?

Originally Posted by nightwyrm

Yeah, that's what I thought. When the math behind THAC0 and AC was created, it was information that should only have been available to the DM. The way THAC0 and AC works makes perfect sense if the DM was the only one with the information.

The player was suppose to roll the dice, add/subtract any modifiers and give that number to the DM. The DM determines the target "to hit" from the PC's THAC0 and the monster's AC (or looks up the chart from 1e) and tells the player whether he hit or missed. The player was never suppose to do anything with his own THAC0. He was suppose to be able to play perfectly fine without even know what his own THAC0 was.

I think Dave Arneson's "Tell me the number, I'll tell you if you hit" quote is applicable in this instance, mainly in support of the idea that - ideally - D&D should be as near as mechanically transparent to the player as possible.

THAC0 should also be a cautionary tale of how legacy systems are sometimes more trouble than they're worth. AC as Armour Type made perfect sense if you had access to OD&D+Chainmail, but became gradually more opaque and esoteric as time (and editions) marched on and explanatory footnotes were excised or misinterpreted.

Re: [AD&D] Why THAC0?

Originally Posted by satorian

At a certain point, it became assumed that players knew their own THAC0. THAC0 had the biggest box on many a TSR character sheet, IIRC. This was true even in AD&D.

I don't doubt that. But when the precursor to THAC0 (the "to hit" charts) was first created, it was intended to be DM-only information. They used the charts to created THAC0 and gave the information to the players but the math it was based on was suppose to facilitate DM usage only.

This is why it's awkward to try to figure out what monster ACs you hit from your rolls and your THAC0. It's suppose to be the other way around.

Re: [AD&D] Why THAC0?

At a certain point, it became assumed that players knew their own THAC0.

That is kinda why they put it in the PHB afterall....

Originally Posted by satorian

In AD&D/2e -10 was an arbitrary floor for AC, and that never made much sense to me.

It is the way Dave made it up for armor based on ships and hull ratings.

Carry over from wargames that Gary kept using cause a simple number line with a limited range made perfect sense to the people that created the game.

What makes no sense to me is things today that still start at AC 10 still being weakest and going up to 50+ AC...why does AC need an unlimited range?

The floor wasn't arbitrary it was designed so allow for ease of functionality. You had a limit of materials you could use to make things from, and even magic had its limits on helping you. The mace may never have touched you from the magical protection, but that force of the blow still knocked you on your duff and caused damage!

What is arbitrary is 3+ editions having everything start at AC 10...what happened to 0-9?

But this is a thread about THAC0 so going any farther will derail outside of the AC system in conjuction with how it worked with THAC0.

Re: [AD&D] Why THAC0?

Originally Posted by bosssmiley

I think Dave Arneson's "Tell me the number, I'll tell you if you hit" quote is applicable in this instance, mainly in support of the idea that - ideally - D&D should be as near as mechanically transparent to the player as possible.

Re: [AD&D] Why THAC0?

I just don't see how that's true. The PHB had THAC0 tables for each class, as well as to hit bonuses from strength. If you knew your weapon's plus or lack thereof, you knew your THAC0. The players were expected to calculate their own THAC0. Now, sure, the DM didn't tell you the AC of the enemy creature, but given a few near misses and near hits, any player could extrapolate the AC. E.g. my Thaco is 5. I miss on a 6 and hit on a 7. The monster has an AC of -2. It's just like D20, but with subtraction. No more or less was hidden from the player. You may be right that it wasn't the original intent to make the THAC0 known to the player, but by AD&D the player was expected to know it and the math behind it.

And yeah, -10 may have come from somewhere but that doesn't make it not arbitrary as used. Bahamut could dip himself in liquid adamantium and let it harden, then wield an enchanted WWII destroyer as a tower shield and he's still have an AC of -10, the same as a decked out high level fighter.

Re: [AD&D] Why THAC0?

Originally Posted by shadzar

What is arbitrary is 3+ editions having everything start at AC 10...what happened to 0-9?

It has to do with the d20. They wanted to assume that an untrained person has a 50/50 chance of hitting an unarmored person. If you fiddle around that chance, you could have some other base AC, but that's what the designers of 3e went with.

Re: [AD&D] Why THAC0?

Do you mean opaque? As in, the players aren't suppose to know the math and mechanics behind the game.

My intent was transparent like a car windscreen (invisible during use, but doing its job anyway). The players are left free to concentrate on play, rather than on juggling numbers.

Opaque? Yeah, that works too, in the rules are a black box to the players. As a player I have no problems with black box rulesets. "Input > witchcraft happens > output" is, IMO, good from the player's perspective.

Re: [AD&D] Why THAC0?

My intent was transparent like a car windscreen (invisible during use, but doing its job anyway). The players are left free to concentrate on play, rather than on juggling numbers.

Opaque? Yeah, that works too, in the rules are a black box to the players. As a player I have no problems with black box rulesets. "Input > witchcraft happens > output" is, IMO, good from the player's perspective.

Yeah, I guess we're just used to different terminologies. To me, transparent has a connotation of having a transparent case or hood and I can see how the engine runs inside the box.

Re: [AD&D] Why THAC0?

Originally Posted by nightwyrm

It has to do with the d20. They wanted to assume that an untrained person has a 50/50 chance of hitting an unarmored person. If you fiddle around that chance, you could have some other base AC, but that's what the designers of 3e went with.

Then completely arbitrary and without real reason, except they couldn't make the math work any other way with the d20 system.

At least with THAC0 you have a closed system that can be explained why THAC0 has the range it does.

Mathematically intuitive would mean d20 started with 0 for AC and got better, and they would make the dice roll comply with that.

Which is simple, d20-10 for the attack roll.

Also why no upper limit to armor? What can give better protection that magic or divine power? Some godslaying metal? Maybe armor made of Sapient Pearwood armor that moves the wearer to avoid an attack or something?

THAC0 can be adjusted to have AC start at 0 and end at 20 without saying roll a d20-10, can 3rd edition? (ignoring 3rd not having the limit of 20 AC for simplicity sake.)

Re: [AD&D] Why THAC0?

Then completely arbitrary and without real reason, except they couldn't make the math work any other way with the d20 system.

Of course it's arbitrary. Lot's of things in games are completely arbitrary. Some in order to make the math work, some just coz the designer felt like it.

Why does the dagger do 1d4 damage, a short sword do 1d6 and a long sword do 1d8? Why are stats based on 3d6 instead of 3d8 or d100? There's no real reason for any of them. The original designers just decided to use what they did instead of something else.

Re: [AD&D] Why THAC0?

Originally Posted by nightwyrm

Of course it's arbitrary. Lot's of things in games are completely arbitrary. Some in order to make the math work, some just coz the designer felt like it.

Why does the dagger do 1d4 damage, a short sword do 1d6 and a long sword do 1d8? Why are stats based on 3d6 instead of 3d8 or d100? There's no real reason for any of them. The original designers just decided to use what they did instead of something else.

I'm not quite sure what your complaint is...

I can partially answer the weapon damage, but that is probably best for another thread...

The point is when looking at THAC0 you see a range including 0. BAB or whatever in 3+ has no 0.

Humans that understand counting know you start at zero and move out.

IE8 was once IE0 when it didn't exist, then IE1, etc.

How is it AC just starts at 10? There is nothing that can mathematically explain it because they could have made it start at 0.

They went to build the system around the dice rather than make a working system and make the dice work for it.

The priority, dice or system, I am calling into question. Making the system work and make sense would have the worst AC being 0, no AC at all. Weak frail humans wearing nothing but fleshy padding called skin.

The inability of the designers is transparent when you can see it was done just because they wanted to use a d20 for everything and couldn't figure a way to make AC fully intuitive.

THAC0 you can see it has a range AND includes that important number....0.

Re: [AD&D] Why THAC0?

The point is when looking at THAC0 you see a range including 0. BAB or whatever in 3+ has no 0.

Humans that understand counting know you start at zero and move out.

ummm....as a historical trivia, zero as a mathematical concept is not as intuitive as we may think: History of Zero

In any case, THAC0 has no 0 either, it goes from 20 to 1. AC goes from 10 to -10 so 0 is a part of the scale, but there's nothing special about AC 0 either, it's just one number in the scale.

The BAB system does have a 0, 0 BAB is what you start with. They can either have the attack bonus start at 0 or the base AC start at 0, not both if you still want something that works. Besides, neither the total attack bonus or AC have a upper or lower cap. You can have a AC lower than 10 if you don't wear armor and have a dex lower than 10.

Both the THAC0 and the BAB system have arbitrary parts to it to make the math work. And don't diss the math. Math working is what makes the whole system run properly.

Re: [AD&D] Why THAC0?

{Scrubbed}

EDIT: Before it's modded away, let me explain why. It's good design philosophy to make things simpler and easier! Just because you think it should all rely on Peano Arithmetic doesn't make it a sound decision to base everything in a game mechanic on number theory or whatever.

Last edited by Roland St. Jude; 2009-06-06 at 05:25 PM.

"It speaks of time as an ally, not as an enemy. It says that patience can sharpen even the smallest of efforts into a weapon that can strike the heart of an empire. Your victories may be small, but over time, a greater victory may be achieved."

Re: [AD&D] Why THAC0?

Originally Posted by shadzar

The priority, dice or system, I am calling into question. Making the system work and make sense would have the worst AC being 0, no AC at all. Weak frail humans wearing nothing but fleshy padding called skin.

I gotta disagree here. AC10 is not the worst AC (it assumes someone with average Dexterity fighting with no drawbacks). Someone who was a blind (-2 AC), cowering (-2 AC) invalid (-5 Dex bonus) with the Vunerable flaw (-1 AC) would have AC 0, as in, anyone would be able to hit them. AC10 is the base AC because 50% of the time, someone unskilled but average will be able to hit someone equally unskilled but average on a d20.

As for THAC0, I really liked it, but then I like a lot of arbitrary mechanics and I've always had a soft spot for 2e.

Armor Class is a misonomer if you want to make the downward scale make sense. Maybe it should be Attack Chance, instead? Someone with AC10 would have a 50% chance of being hit by someone who is unskilled, but someone with a -1 AC would have no chance of being hit by someone unskilled.

if d20 + AC > Thac0 you hit. It's... pretty simple, really. Not the most intuitive, but hey. And then those plusses and minuses? You add bonuses to your roll. I don't get what all the problem is.

Re: [AD&D] Why THAC0?

I actually think THAC0 is more intuitive than BAB/adjusted ability scores in some respects - it is a simple statement of what you need to roll to succeed in a particular situation (an average character wearing full plate armour and carrying a shield, IIRC).

AC is really an explicit version of the ad-hoc modifiers that a DM might want to apply to things like ability checks and thief abilities.

In reality, if the various mechanics for doing things had been made more consistent (leaving percentiles for situations where the only factor is chance), the end result would have been perfectly workable.

Re: [AD&D] Why THAC0?

Originally Posted by nightwyrm

I always thought the reason THAC0 existed was to keep information away from the players. The players weren't suppose to know the AC of the monsters they're facing. If I recall correctly (and I probably aren't), the original intention was that players aren't even suppose to know their own THAC0. There was a reason why the THAC0 chart was on the DM screen. The DM was suppose to be the only one to figure out the target "to hit" number based on the PC's THAC0 and the monster's AC because he was the only one with both pieces of information. THAC0 was there to maintain the illusion so the PCs can't "peek under the hood" so to speak. The only thing the PCs were suppose to know after rolling their dice was "hit or miss".

This is the only reason I could think of to explain the roundabout nature of THAC0.

THAC0 itself is a shorthand for the attack tables, which were set up the way they were for two reasons: 1) For continuity between OD&D and AD&D (as Gygax mentions in the DMG), 2) On account of the "five repeating twenties" rule, which was the precursor to "20 is always a hit" (in 2e).

Originally Posted by shadzar

Originally Posted by nightwyrm

Originally Posted by Gaurd Juris

If I'm not mistaken, in original 1e D&D, there was no thaco or anything like it. Instead, you rolled a d20 and consulted a table which had every single monster on one axis and the player character levels on the other. I'm not 100% sure on the specifics.

What I do know is that when 2e came around, THAC0 was developed as an abstraction of this graph. Because they operated on the same function as the chart, THAC0 and AC end up decreasing as you become more formidable.

Yeah, that's what I thought. When the math behind THAC0 and AC was created, it was information that should only have been available to the DM. The way THAC0 and AC works makes perfect sense if the DM was the only one with the information.

The player was suppose to roll the dice, add/substract any modifiers and give that number to the DM. The DM determines the target "to hit" from the PC's THAC0 and the monster's AC (or looks up the chart from 1e) and tells the player whether he hit or missed. The player was never suppose to do anything with his own THAC0. He was suppose to be able to play perfectly fine without even know what his own THAC0 was.

It is exactly the same thing as THAC0 but takes up 3 pages rather than the little amount of space the THAC0 table takes up.

Actually, in this case you are all three mistaken. THAC0 does appear in the first edition DMG in the appendix. All Monsters are listed there with their THAC0 values for convenience. So, THAC0 was in the game from at least 1979.

Originally Posted by nightwyrm

Originally Posted by Chronos

That's still the case. In 3.x, a player can roll the die and add all appropriate modifiers, and say "I got a 19". The DM then looks at the monster's AC, sees whether it's higher than 19, and tells the player whether the attack hit. The player doesn't need to know the monster's AC any more than in 2nd edition. You might be able to figure it out ("Wait, last round, I missed on an 18, and this round I hit on a 19"), but you could do that in 2nd, too.

But the 3.x player knows his own BAB. He can figure out that the monster's AC was 19. If the 2e/1e player weren't suppose to know his own THAC0, then even if the player hit on a 19, he can't figure out what the real AC of the monster was because he doesn't know his own THAC0.

All players should know their own THAC0 in second edition, though this may not be the case in first edition. The tongue in cheek introduction suggests that players peeking into the DMG should be severely chastised, but in actual fact anybody interested in the maths would have soon known about Fighting Ability/THAC0.

Originally Posted by satorian

At a certain point, it became assumed that players knew their own THAC0. THAC0 had the biggest box on many a TSR character sheet, IIRC. This was true even in AD&D.

There was nothing wrong with THAC0. D20 makes a minor improvement, making hitting stuff slightly more intuitive. It's not massively better, but it is a little bit better. Hm, I guess there is one substantive improvement. In AD&D/2e -10 was an arbitrary floor for AC, and that never made much sense to me. D20 did away with that (though many tables had already house ruled it away anyway).

In Original Dungeons & Dragons the range was 9 to 2. The reason that −10 is the floor in AD&D is because Full Plate +5 and a Shield +5 = AC −10. It is literally the best AC you can achieve with armour (and shield was counted as armour), but it doesn't actually prohibit lower values, as can be seen in certain Dragons in 2e. Also, a range of 10 to −10 just seems good.

Originally Posted by bosssmiley

THAC0 should also be a cautionary tale of how legacy systems are sometimes more trouble than they're worth. AC as Armour Type made perfect sense if you had access to OD&D+Chainmail, but became gradually more opaque and esoteric as time (and editions) marched on and explanatory footnotes were excised or misinterpreted.

Indeed. The further you get from the initial idea the less it seems to "make sense."

Originally Posted by satorian

I just don't see how that's true. The PHB had THAC0 tables for each class, as well as to hit bonuses from strength. If you knew your weapon's plus or lack thereof, you knew your THAC0. The players were expected to calculate their own THAC0. Now, sure, the DM didn't tell you the AC of the enemy creature, but given a few near misses and near hits, any player could extrapolate the AC [e.g. my THAC0 is 5. I miss on a 6 and hit on a 7]. The monster has an AC of −2. It's just like D20, but with subtraction. No more or less was hidden from the player. You may be right that it wasn't the original intent to make the THAC0 known to the player, but by AD&D the player was expected to know it and the math behind it.

10 may have come from somewhere but that doesn't make it not arbitrary as used. Bahamut could dip himself in liquid adamantium and let it harden, then wield an enchanted WWII destroyer as a tower shield and he's still have an AC of −10, the same as a decked out high level fighter.

You can certainly work out your modified THAC0 in 2e, though it is a bit of a pain to deal with inverted modifiers.

Originally Posted by nightwyrm

In any case, THAC0 has no 0 either, it goes from 20 to 1. AC goes from 10 to −10 so 0 is a part of the scale, but there's nothing special about AC 0 either, it's just one number in the scale.

The BAB system does have a 0, 0 BAB is what you start with. They can either have the attack bonus start at 0 or the base AC start at 0, not both if you still want something that works. Besides, neither the total attack bonus or AC have a upper or lower cap. You can have a AC lower than 10 if you don't wear armor and have a dex lower than 10.

Both the THAC0 and the BAB system have arbitrary parts to it to make the math work. And don't diss the math. Math working is what makes the whole system run properly.

Yes, indeed. Both are entirely arbitrary scales on which to measure hit chances.

Originally Posted by lesser_minion

Originally Posted by Chronos

Ultimately, using THAC0 gives exactly the same effect as the system in 3.x, but the 3.x system is simpler and easier to use. OK, maybe it's only a little bit simpler, but it does have an advantage over THAC0, and I've never seen anyone argue that THAC0 has any advantage at all.

I actually think THAC0 is more intuitive than BAB/adjusted ability scores in some respects - it is a simple statement of what you need to roll to succeed in a particular situation (an average character wearing full plate armour and carrying a shield, IIRC).

AC is really an explicit version of the ad-hoc modifiers that a DM might want to apply to things like ability checks and thief abilities.

In reality, if the various mechanics for doing things had been made more consistent (leaving percentiles for situations where the only factor is chance), the end result would have been perfectly workable.

Until very recently I would have agreed with Chronos that there is just no advantage to the descending THAC0 system, and also that the AB system is a bit simpler and subjectively more intuitive (my girlfriend found AB bewildering compared to THAC0, and that is just the way her mind is set up, preferring small numbers I suppose or something). In recent months I have come to see why THAC0 and the descending Armour Class system might have a slight edge, and the reason is basically for totalling defence and attack modifiers. You never need to convert them when applying them to the dice roll, a −3 is always a −3 and a +4 is always a +4. The next step is eliminating THAC0 altogether in favour of Fighting Ability, which is essentially BAB or your fighter level. As a hybrid, I like that a lot.

Good discussion, I am enjoying it!

Last edited by Matthew; 2009-06-06 at 08:35 PM.

It is a joyful thing indeed to hold intimate converse with a man after one’s own heart, chatting without reserve about things of interest or the fleeting topics of the world; but such, alas, are few and far between.

Re: [AD&D] Why THAC0?

Originally Posted by Matthew

Actually, in this case you are all three mistaken. THAC0 does appear in the first edition DMG in the appendix. All Monsters are listed there with their THAC0 values for convenience. So, THAC0 was in the game from at least 1979.

Before I did it out again from the closet, tell me what page number?

Originally Posted by DMG glossary

Surprise- Both parties in an encounter must check to see if either or both are surprised, which may result in a loss of initiative (q.v.).

To Hit -That number which must be matched or beaten by a 20-sided die roll in order to inflict damage on an opponent.

Trap - Any of several mechanical or magical devices which may be triggered by adventurers, usually causing damage to one or more of them. Examples
are pits, pits with spikes, poison needle traps on treasure chests, etc.

Re: [AD&D] Why THAC0?

Originally Posted by shadzar

Before I did it out again from the closet, tell me what page number?

No THAC0 definition.

Pages 196 to 214, third column: "To Hit A.C.0"

It is a joyful thing indeed to hold intimate converse with a man after one’s own heart, chatting without reserve about things of interest or the fleeting topics of the world; but such, alas, are few and far between.